Non-conductive materials, such as paper, plastic and glass, are substantially microwave transparent as they neither reflect nor absorb microwaves. While containers for some food in these "transparent" materials are acceptable when the foods are heated, surfaces of foods heated in a microwave oven tend to become soggy due to condensation of water vapor on the food surface. This is a problem for foods that are desirably crisp such as fried chicken, french fries, pizza and the like. As a consequence, metallized polyester films have been developed as susceptors (thin films of a conductive material designed to absorb microwave energy) in order to heat food adjacent the metallized film area by conduction and radiation. The metallized area, because of its low heat mass, heats quickly and transfers the heat for browning and crisping. Thus, susceptors assist in heating food surfaces to prevent water vapor condensation and soggy food surfaces.
Thin coatings of aluminum on polyester film are known and used as such susceptors by adhesive lamination to paper or paperboard. However, aluminum oxidizes over time, thus losing its conductivity, and aluminum also continues to absorb microwave energy. This creates the possibility of overheating. Controlling the amount of metal deposited on the polyester is also difficult. The adhesive lamination can pose problems due to outgassing from the adhesive during heating which causes odors and undesirable tastes.